1. Field of the Invention
This invention relates to a normally-closed electromagnetic valve in which a moving core is urged toward a valve seat, so that a valve element which is movable forward and backward together with the moving core abuts against the valve seat to thereby achieve a valve-closed condition. The present invention also relates to a manufacturing method for the normally-closed electromagnetic valve.
2. Description of the Related Art
There is known a normally-closed electromagnetic valve working that a moving core is urged by a resilient member such as a spring so that a valve element, which is provided at a distal end portion of the moving core, abuts against a valve seat to thereby achieve a valve-closed condition, and exciting a core by an energization coil so that the moving core is attracted toward the fixed core by an electromagnetic attracting force thereof to thereby achieve a valve-open condition (see Japanese Patent Unexamined Publication no. JP-A-8-152076).
In such a normally-closed electromagnetic valve, the urging force, applied to the moving core from the resilient member, is an important element for determining the valve-closing force of the electromagnetic valve. Therefore, when there is a variation in characteristics of the resilient member used in the normally-closed electromagnetic valve, a variation develops in the urging force of the resilient member in the closed condition of the valve. Namely, at the time of energization (excitation), a variation develop in the electromagnetic attracting force required for opening the valve against the bias of the resilient urging force, and at the time of de-energization (excitation), a variation develops in the valve-opening force (valve opening characteristics) for opening the valve against the bias of the resilient member, and this leads to a variation in pressure regulation control characteristics. Thus, the variation in the characteristics of the resilient member has been a hindrance to the enhanced precision of the pressure regulation control.
In the normally-closed electromagnetic valve, it is required that a flow rate or a pressure differential at the time of opening the valve should be set to a target value. Therefore, in the production of the normally-closed electromagnetic valve, it is required that a load, which is applied from a spring so as to close the valve, and an electromagnetic attracting force should be set to a predetermined value.
However, it is difficult to eliminate a manufacturing tolerance and an assembling error with respect to all of the component parts. On the other hand, even when there are a variation in the load applied from the spring and a variation in electromagnetic attracting force, the flow rate or the pressure differential at the time of opening the valve can be set to a target value by adjusting a stroke of a moving core. See Japanese Patent Examined Publication JP-B-3,444,151 and Japanese Patent Unexamined Publication JP-A-2004-506573